Type 1 diabetes (T1D) is a chronic disease caused by the autoimmune destruction of the insulin producing beta cells in the pancreas. Current treatment options for T1D are severely limited. All patients diagnosed with T1DM are placed on life-long insulin therapy and the current standard of care for adult patients with T1DM is intensive diabetes therapy to avoid long-term medical complications such as diabetic nephropathy, neuropathy, and retinopathy, among other complications (1, 2). T cells are believed to play a major role in the autoimmune destruction of pancreatic ? cells in T1D (3, 4). Many aspects of immune regulation including regulatory T cell (Treg) function has been found to be defective in T1D (4-10). Hence, restoration of Treg numbers and function to normalcy can be an effective strategy in preventing and/or treating T1D (11-14). Several TCR activation dependent methods have been developed for Treg expansion but they cannot be used in vivo, because such methods can activate Teff as well as Tregs, thus limiting the therapeutic efficacy. Methods for TCR-dependent ex vivo Treg expansion have been developed, but is impractical as the Tregs will have to be patient specific and requires repeated ex vivo expansion. Our recent studies have shown that soluble OX40L and Jagged-1 (Jag1) can cause a significant expansion of Tregs in vivo and suppress T1D in NOD mice. However, developing a clinically acceptable therapeutic consisting of two soluble ligands may face regulatory hurdles. We have developed mouse (m) and human (h) OX40L-Jag1- Fc fusion proteins that can be used to expand Tregs ex vivo. In our Phase-1 studies we will establish their therapeutic potential by testing their abilityto induce Tregs in vivo through the following aims: In aim-1, we will test if mOX40L-Jag1-Fc fusion protein can expand Tregs and delay/prevent the onset of hyperglycemia in NOD mice. In aim-2, we will test if hOX40L-Jag1-Fc is able to expand human Tregs in immunodeficient NSG mice reconstituted with human immune cells. Successful outcome of our studies would illustrate the potential clinical utility of our novel therapeutic. Subsequently, Absorption, Distribution, Metabolism, Excretion and Toxicity studies can be carried out to establish the safety of OX40L-Jag1-Fc required for IND filing to FDA. This novel therapeutic can be potentially used to effectively treat T1D and other autoimmune diseases with Treg insufficiency.